Additive manufacturing techniques and processes generally involve the buildup of one or more materials, e.g., layering, to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), additive manufacturing encompasses various manufacturing and prototyping techniques known under a variety of names, including, e.g., freeform fabrication, 3D printing, rapid prototyping/tooling, etc. Additive manufacturing techniques may be used to fabricate simple or complex components from a wide variety of materials. For example, a freestanding object may be fabricated from a computer-aided design (CAD) model.
A particular type of additive manufacturing is commonly known as 3D printing. One such process commonly referred to as Fused Deposition Modeling (FDM), or Fused Layer Modeling (FLM), comprises melting a thin layer of thermoplastic material, and applying this material in layers to produce a final part. This is commonly accomplished by passing a continuous thin filament of thermoplastic material through a heated nozzle, or by passing thermoplastic material into an extruder, with an attached nozzle, which melts the thermoplastic material and applies it to the structure being printed, building up the structure. The heated material may be applied to the existing structure in layers, melting and fusing with the existing material to produce a solid finished part.
The filament used in the aforementioned process may be produced, for example, by using a plastic extruder. This plastic extruder includes a steel screw configured to rotate inside of a heated steel barrel. Thermoplastic material in the form of small pellets may be introduced into one end of the rotating screw. Friction from the rotating screw, combined with heat from the barrel, may soften the plastic, which may then be forced under pressure through a small round opening in a die that is attached to the front of the extruder barrel. In doing so, a “string” of material may be extruded, after which the extruded “string” of material may be cooled and coiled up for use in a 3D printing machine or other additive manufacturing system.
Melting a thin filament of material in order to 3D print an item may be a slow process, which may be suitable for producing relatively small items or a limited number of items. The melted filament approach to 3D printing may be too slow to manufacture large items. However, the fundamental process of 3D printing using molten thermoplastic materials may offer advantages for the manufacture of larger parts or a larger number of items.
In some instances, the process of 3D printing a part may involve a two-step process. For example, the process may utilize a large print bead to achieve an accurate final size and shape. This two-step process, commonly referred to as near-net-shape, may begin by printing a part to a size slightly larger than needed, then machining, milling or routing the part to the final size and shape. The additional time required to trim the part to a final size may be compensated for by the faster printing process.
A common method of additive manufacturing, or 3D printing, generally may include forming and extruding a bead of flowable material (e.g., molten thermoplastic), applying the bead of material in a strata of layers to form a facsimile of an article, and machining the facsimile to produce an end product. Such a process may be achieved using an extruder mounted on a computer numeric controlled (CNC) machine with controlled motion along at least the x, y, and z-axes. In some cases, the flowable material, such as, e.g., molten thermoplastic material, may be infused with a reinforcing material (e.g., strands of fiber or a combination of materials) to enhance the material's strength.
The flowable material, while generally hot and pliable, may be deposited upon a substrate (e.g., a mold), pressed down or otherwise flattened to some extent, and leveled to a consistent thickness, preferably by means of a tangentially compensated roller mechanism. The compression roller may be mounted in or on a rotatable carriage, which may be operable to maintain the roller in an orientation tangential, e.g., perpendicular, to the deposited material (e.g., bead or beads). In some embodiments, the compression roller may be smooth and/or solid. The flattening process may aid in fusing a new layer of the flowable material to the previously deposited layer of the flowable material. The deposition process may be repeated so that successive layers of flowable material are deposited upon an existing layer to build up and manufacture a desired component structure. In some instances, an oscillating plate may be used to flatten the bead of flowable material to a desired thickness; thus, effecting fusion to the previously deposited layer of flowable material. The deposition process may be repeated so that successive layers of flowable material are deposited upon an existing layer to build up and manufacture a desired component structure. In order to achieve proper bonding between printed layers, the temperature of the layer being printed upon must be within a certain range. It must be cool enough to have solidified sufficiently to support the pressures generated by the application of the new layer but must also be warm enough to soften and fuse with the new layer.
As a layer is printed, it begins to cool. The rate at which the layer cools depends on a number of factors such as, for example, the thermal characteristics of the material being printed, the temperature at which it is being printed, the ambient temperature of the area where the printing process is being conducted, and air flow. Many of these factors will not necessarily be consistent between different print jobs or even consistent during a single print job. Moreover, each layer of printed material requires a certain amount of time from the time the material is deposited until it has cooled sufficiently to accept (e.g., to appropriately bond with and structurally support) another layer. This amount of time is referred to herein as the “minimum cooling time per layer”. Because of the variability of the above factors, the minimum cooling time per layer, even for a specific polymer, can vary.
It is generally desirable to print a part as quickly as possible, and as such, a reduction in the minimum cooling time per layer may be beneficial. In some cases, a 3D printer may be able to print a layer faster than the minimum cooling time per layer. Thus, the minimum cooling time per layer may negatively affect the rate at which the 3D printer can print a part without error. One approach to decrease the time required to produce a part may include printing each layer at a print speed corresponding with the minimum cooling time per layer.
Typical 3D printed parts may have varying geometry at different print heights so that the length of the print bead may be different on different layers. Further, due to the geometric characteristics of the part being printed, different groups of layers within the part may require different amounts of time to cool to a suitable temperature even if all other factors are constant. Therefore, the minimum cooling time per layer may not be the same for every layer of the same part. As such, an ideal print speed required to continuously print a part may differ for different layers.
The speed at which a 3D printing machine, e.g., a CNC machine, operates may be specified within a program (e.g., a CNC program) that defines the motions of the machine required to print a desired part. The program may be developed using a computer system running appropriate software separate from the 3D printing machine. The program may be developed at a location separate from the 3D printing machine, and prior to the intended use of the program. Thus, the exact parameters of the print environment where the program will be executed may not be known when the program is developed. Accordingly, determining optimal print speed for each print layer during the execution of the program may be difficult. To accommodate for a range of different print environments, the program may be developed using nominal parameters and used to determine an approximate print speed recognizing that additional factors encountered during the print process may require print speeds that vary from the programmed print speeds.
Therefore, it may be desirable to adjust the pre-programmed print speed during the print process itself. One method of adjusting the print speed may include adding a feature to a controller of the 3D printing machine which allows the operator to manually vary the speed at which all motions of the 3D printing machine are executed by a fixed amount. But such a method has several drawbacks. First, consistent attention by the operator is required to maintain a proper layer-to-layer cycle time as the size of the printed layers may vary during the print process. Second, certain motions during a start-stop sequence on each layer may require a specific machine speed, and because varying the overall speed of motion for the program may also vary these speeds, the 3D printing machine may not function properly during the start-stop sequence at the adjusted speed.
Another factor that may be considered when adjusting the pre-programmed print speed is the minimum speed at which the 3D printing machine can operate during the print process, e.g., “the minimum print speed.” For example, a 3D printing machine may be limited by the mechanical or electrical capabilities of the components (e.g., servomotors) of the 3D printing machine such that the 3D printing machine cannot deposit material slower than a specific rate, e.g., the minimum print speed. On small parts, the 3D printing machine may not be able to print continuously if printing at the minimum print speed completes a layer in less time than is required to sufficiently cool the layer to accept the next layer. In other words, in order to produce a continuously printed part, a print speed of the 3D printing process may not be adjusted to a rate equal to the minimum print speed if the minimum print speed is less than the minimum cooling time per layer. In such a case, a pause may be required to enable sufficient cooling time between layers.